Acceleration sensor

ABSTRACT

An acceleration sensor includes a weight portion having a recess section and a solid section, beam portions, a movable electrode provided on the opposite surface of the weight portion from an open surface of the recess section to extend over the recess section and the solid section, a first fixed electrode arranged at the opposite side of the movable electrode from the recess section, and a second fixed electrode arranged at the opposite side of the movable electrode from the solid section. The acceleration sensor detects acceleration using a change in capacitance between the movable electrode and the fixed electrodes caused by rotation of the weight portion. The beam portions are shifted toward the recess section such that an angle between a perpendicular line extending from a gravity center position of the weight portion to the rotation axis and a surface of the movable electrode becomes equal to 45 degrees.

FIELD OF THE INVENTION

The present invention relates to a capacitance-type acceleration sensor.

BACKGROUND OF THE INVENTION

There is conventionally known an acceleration sensor that includes, asshown in FIG. 5A, a rectangular parallelepiped weight portion 100 havinga movable electrode, a pair of beam portions 101 for rotatablysupporting the weight portion 100 substantially at a center in thelongitudinal direction of the weight portion 100 and a pair of fixedelectrodes 102 and 103 arranged in a spaced-apart opposing relationshipwith respect to one side and the other side of the surface of the weightportion 100 demarcated by a straight border line interconnecting thebeam portions 101 (see, e.g., Patent Document 1). This accelerationsensor detects the acceleration applied to the weight portion 100 bydifferentially detecting the change in capacitance between the movableelectrode (the section of the weight portion 100 facing the fixedelectrodes 102 and 103) and the fixed electrodes 102 and 103 caused bythe rotation of the weight portion 100 about the border line as arotation axis. In this acceleration sensor, a recess portion 104 isformed at one side (the right side in FIG. 5A) of the rear surface ofthe weight portion 100 with respect to the border line so that theweight of the weight portion 100 can become different at one side (theright side) and the other side (the left side) thereof with respect tothe border line. Therefore, upon applying acceleration, the momentacting about the border line as a rotation axis is generated in theweight portion 100. In order to prevent the section of the weightportion 100 having the recess portion 104 from being deformed by theambient stresses, a reinforcing wall 105 for bisecting the recessportion 104 is one-piece formed with the weight portion 100 to extend inthe direction parallel to the border line.

-   [Patent Document 1] Japanese Patent Application Publication No.    2008-544243

The acceleration sensor stated above is capable of detectingacceleration in two directions orthogonal to the rotation axis. Thedetection sensitivity in the two directions is equalized by setting theangle θ between the perpendicular line extending from the gravity centerposition of the weight portion 100 to the rotation axis and the surfaceof the weight portion 100 to become equal to about 45 degrees. In thisregard, a method of increasing the area of the movable electrode isadoptable as one means for enhancing the detection sensitivity of theacceleration sensor. If this method is employed, the thickness of theweight portion 100 needs to be increased in order to keep the angle θ atabout 45 degrees. This method is not realistic because the increase inthe thickness of the weight portion 100 prolongs the duration of anetching step for forming the weight portion 100.

In order to keep the angle θ at about 45 degrees without increasing thethickness of the weight portion 100, it is thinkable to employ a methodin which the weight of the weight portion 100 is reduced by cutting awaythe section of the weight portion 100 existing just below the beamportion 101 as shown in FIG. 55. Use of this method, however, poses aproblem in that the weight-reducing thin section of the weight portion100 is insufficient in strength.

SUMMARY OF THE INVENTION

In view of the above, the present invention provides an accelerationsensor capable of enhancing detection sensitivity without having toincrease the thickness of a weight portion or to reduce the weight ofthe weight portion.

In accordance with a first aspect of the present invention, there isprovided an acceleration sensor, including: a sensor unit including aweight portion having a recess section with one open surface and a solidsection one-piece formed with the recess section, a pair of beamportions configured to rotatably support the weight portion in such astate that the recess section and the solid section are arranged along arotation direction, a movable electrode provided on the opposite surfaceof the weight portion from the open surface of the recess section toextend over the recess section and the solid section, a first fixedelectrode arranged at the opposite side of the movable electrode fromthe recess section and a second fixed electrode arranged at the oppositeside of the movable electrode from the solid section, the accelerationsensor being configured to detect acceleration based on a change incapacitance between the movable electrode and the fixed electrodescaused by rotation of the weight portion about a rotation axis definedby a straight line interconnecting the beam portions, wherein the beamportions are shifted toward the recess section such that an anglebetween a perpendicular line extending from a gravity center position ofthe weight portion to the rotation axis and a surface of the movableelectrode becomes substantially equal to 45 degrees.

With such configuration, the increase of the area of the movableelectrode and the resultant enhancement of the detection sensitivity canbe realized by merely shifting the beam portions toward the first recesssection such that an angle between a perpendicular line extending from agravity center position of the weight portion to the rotation axis and asurface of the movable electrode becomes substantially equal to 45degrees. It is therefore possible to enhance the detection sensitivitywithout having to increase the thickness of the weight portion or toreduce the weight of the weight portion.

In addition, a second recess section with one open surface may beprovided in the first solid section of the weight portion. An auxiliaryweight portion made of a metallic material may be embedded in the secondrecess section. By embedding the auxiliary weight portion in the secondrecess section, it is possible to reduce the size of the weight portionwhile maintaining the weight balance of the weight portion.Consequently, it is possible to reduce the overall size of theacceleration sensor.

In accordance with a second aspect of the present invention, there isprovided an acceleration sensor, including: a sensor unit including aweight portion having a recess section with one open surface and a solidsection one-piece formed with the recess section, a pair of beamportions configured to rotatably support the weight portion in such astate that the recess section and the solid section are arranged along arotation direction, a movable electrode provided on the opposite surfaceof the weight portion from the open surface of the recess section toextend over the recess section and the solid section, a first fixedelectrode arranged at the opposite side of the movable electrode fromthe recess section and a second fixed electrode arranged at the oppositeside of the movable electrode from the solid section, the accelerationsensor being configured to detect acceleration based on a change incapacitance between the movable electrode and the fixed electrodescaused by rotation of the weight portion about a rotation axis definedby a straight line interconnecting the beam portions; and a first fixedplate arranged in a spaced-apart relationship with the surface of theweight portion facing the fixed electrodes, the fixed electrodesprovided on one surface of the first fixed plate, wherein protrusionsare formed on the surface of the movable electrode facing the fixedelectrodes and wherein engraving sections are formed in the areas of thefixed electrodes facing the protrusions by digging out one surface ofthe first fixed plate.

In accordance with a third aspect of the present invention, there isprovided an acceleration sensor, including: a sensor unit including aweight portion having a recess section with one open surface and a solidsection one-piece formed with the recess section, a pair of beamportions configured to rotatably support the weight portion in such astate that the recess section and the solid section are arranged along arotation direction, a movable electrode provided on the opposite surfaceof the weight portion from the open surface of the recess section toextend over the recess section and the solid section, a first fixedelectrode arranged at the opposite side of the movable electrode fromthe first recess section and a second fixed electrode arranged at theopposite side of the movable electrode from the solid section, theacceleration sensor being configured to detect acceleration based on achange in capacitance between the movable electrode and the fixedelectrodes caused by rotation of the weight portion about a rotationaxis defined by a straight line interconnecting the beam portions,wherein protrusions are formed on the surfaces of the fixed electrodesfacing the movable electrode.

With such configuration, even if an impact great enough to bring theprotrusions into contact with the fixed electrodes is applied to theacceleration sensor, the protrusions come into contact with the firstfixed plate through the engraving sections. Thus the protrusions do notmake direct contact with the fixed electrodes. It is therefore possibleto prevent the protrusions from adhering to the fixed electrodes.

In accordance with a fourth aspect of the present invention, there isprovided an acceleration sensor, including: a sensor unit including aweight portion having a recess section with one open surface and a solidsection one-piece formed with the recess section, a pair of beamportions configured to rotatably support the weight portion in such astate that the recess section and the solid section are arranged along arotation direction, a movable electrode provided on the opposite surfaceof the weight portion from the open surface of the recess section toextend over the recess section and the solid section, a first fixedelectrode arranged at the opposite side of the movable electrode fromthe recess section, a second fixed electrode arranged at the oppositeside of the movable electrode from the solid section and a pair ofelectrode portions having detection electrodes electrically connected tothe fixed electrodes, the acceleration sensor being configured to detectacceleration based on a change in capacitance between the movableelectrode and the fixed electrodes caused by rotation of the weightportion about a rotation axis defined by a straight line interconnectingthe beam portions, wherein the sensor unit includes two sensor unitsformed in a single chip, the electrode portions are arranged along onedirection to divide the chip into two halves, the weight portions of thetwo sensor units are arranged in point symmetry with respect to a centerof an array of the electrode portions, and the beam portions arearranged such that a straight line interconnecting the beam portionsextends in a direction orthogonal to the arranging direction of theelectrode portions.

With such configuration, the symmetry of the acceleration sensor as awhole is enhanced. Therefore, even when the acceleration sensor isdistorted by thermal expansion or other causes, the distortion isuniformly generated in the entirety of the acceleration sensor. Thus theoverall balance is not impaired. It is therefore possible to increasethe accuracy of the output temperature characteristics. Moreover, thedistances between the electrode portions and the weight portions becomeequal to each other and, therefore, the distances between the electrodeportions and the fixed electrodes get equalized. This makes it possibleto equalize the wiring lengths of the respective conductive patternsinterconnecting the electrodes. Accordingly, it is possible to reducethe difference in parasitic capacitance of the respective conductivepatterns and to reduce the difference in capacitance between the movableelectrodes and the fixed electrodes. In addition, it is possible toincrease the distance from the beam portions to the longitudinal ends ofthe weight portions, namely the rotation radii of the weight portions.This makes it possible to reduce the rotational displacement requiredfor the sensor chip to obtain the same detection sensitivity as providedby the conventional acceleration sensor of the same size. Accordingly,it is possible to increase the bending strength of the beam portions.Even if the movable electrodes adhere to the fixed electrodes, it ispossible to detach the movable electrodes from the fixed electrodesusing the restoration force of the beam portions.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects and features of the present invention will become apparent fromthe following description of preferred embodiments given in conjunctionwith the accompanying drawings.

FIG. 1 is a partial section view showing an acceleration sensoraccording to a first embodiment of the present invention.

FIG. 2 is an exploded perspective view of the acceleration sensor.

FIG. 3A is a top view of the acceleration sensor with an upper fixingplate and a conductive pattern removed for clarity and FIG. 3B is asection view taken along line A-A′ in FIG. 3A.

FIG. 4 is a bottom view showing a sensor chip of the accelerationsensor.

FIG. 5A is a partial section view showing a conventional accelerationsensor and FIG. 5B is a partial section view of the conventionalacceleration sensor in which the section existing just below a beamportion is cut away.

FIG. 6 is a section view showing an acceleration sensor according to asecond embodiment of the present invention.

FIG. 7A is a partial section view showing an acceleration sensoraccording to a third embodiment of the present invention and FIG. 7B isa partial section view of an acceleration sensor according to a modifiedexample of the third embodiment.

FIG. 8A is a partial section view showing an acceleration sensoraccording to a fourth embodiment of the present invention and FIG. 8B isa partial section view of an acceleration sensor according to a modifiedexample of the fourth embodiment.

FIG. 9 is a partial section view showing an acceleration sensoraccording to a reference example of the present invention.

FIG. 10 is an exploded perspective view showing an acceleration sensoraccording to a fifth embodiment of the present invention.

FIG. 11 is a bottom plan view showing a sensor chip of the accelerationsensor of the fifth embodiment.

FIG. 12 is a section view of the acceleration sensor of the fifthembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of an acceleration sensor according to the present inventionwill now be described in detail with reference to the accompanyingdrawings. Throughout the drawings, identical or similar portions will bedesignated by like reference symbols with no description made thereon.In the following description, the vertical direction in FIG. 1 will bedefined as an up-down direction, the direction parallel to thetransverse direction of a sensor chip 1 as an x-direction, the directionparallel to the longitudinal direction of the sensor chip 1 as ay-direction and the direction orthogonal to the x-direction and they-direction as a z-direction.

First Embodiment

As shown in FIGS. 1 and 2, an acceleration sensor according to a firstembodiment includes a sensor chip 1, an upper fixed plate 2 a fixed tothe upper surface of the sensor chip 1 and a lower fixed plate 2 b fixedto the lower surface of the sensor chip 1.

The sensor chip 1 includes a frame portion 3 having two rims 3 a and 3 bformed into a rectangular shape when seen in the up-down direction andarranged side by side along the longitudinal direction, rectangularparallelepiped weight portions 4 and 5 arranged adjacent to each otherinside the rims 3 a and 3 b in a spaced-apart relationship with respectto the inner circumferential surfaces of the rims 3 a and 3 b, two pairsof beam portions 6 a, 6 b, 7 a and 7 b for interconnecting the innercircumferential surfaces of the rims 3 a and 3 b and the side surfacesof the weight portions 4 and 5 to rotatably support the weight portions4 and 5 with respect to the frame portion 3, and movable electrodes 4 aand 5 a formed on the upper surfaces of the weight portions 4 and 5.

As shown in FIGS. 1 and 3B, each of the weight portions 4 and 5 includesa recess section 41 or 51 opened in one surface (the lower surface)thereof and a solid section 40 or 50 one-piece formed with the recesssection 41 or 51. The recess section 41 or 51 is formed to have arectangular shape when seen in a plan view in the direction normal tothe open surface (in the up-down direction). Reinforcing walls 42 or 52for dividing the inside of the recess section 41 or 51 into four spacesare one-piece formed with each of the weight portions 4 and 5.

In the present embodiment, as shown in FIG. 4, the acceleration sensoremploys a configuration in which the central portions of the reinforcingwalls 42 or 52 having a chevron shape when seen in a plan view areinterconnected by a flat reinforcing wall 42′ or 52′, namely aconfiguration in which the reinforcing walls 42 or 52 are connected tothe inner wall surface in the positions where the reinforcing walls 42or 52 do not intersect the corners of the recess section 41 or 51.Accordingly, the angle between the reinforcing walls 42 or 52 and theinner wall surface becomes obtuse at four corners of the recess section41 or 51. This makes it easy to form (etch) the recess section 41 or 51in the weight portions 4 and 5.

One pair of the beam portions 6 a and 6 b interconnects the rim 3 a andthe substantially central sections in the x-direction of the sidesurfaces of the weight portion 4 facing the rim 3 a. Similarly, anotherpair of the beam portions 7 a and 7 b interconnects the rim 3 b and thesubstantially central sections in the x-direction of the side surfacesof the weight portion 5 facing the rim 3 b. Accordingly, the straightline interconnecting the beam portions 6 a and 6 b and the straight lineinterconnecting the beam portions 7 a and 7 b become rotation axes aboutwhich the weight portions 4 and 5 rotate, respectively.

The sensor chip 1 is formed by processing a SOI (Silicon-On-Insulator)substrate by a semiconductor fine processing technology. The sectionsincluding the upper surfaces of the weight portions 4 and 5 become themovable electrodes 4 a and 5 a. Protrusions 43 a, 43 b, 53 a and 53 bfor preventing the weight portions 4 and 5 from directly colliding withthe upper fixed plate 2 a and the lower fixed plate 2 b are provided toprotrude from the upper and lower surfaces of the weight portions 4 and5.

In this regard, if the protrusions 43 a, 43 b, 53 a and 53 b are formedof the main material of the sensor chip 1 such as a silicon film or asilicon oxide film, it becomes easy to form the protrusions 43 a, 43 b,53 a and 53 b. The surface layers of the protrusions 43 a, 43 b, 53 aand 53 b may be coated with a carbon material. In this case, it ispossible to increase the mechanical strength of the protrusions 43 a, 43b, 53 a and 53 b and to prevent the protrusions 43 a, 43 b, 53 a and 53b from being damaged by the collision with the upper fixed plate 2 a andthe lower fixed plate 2 b. If carbon nano tubes are used as the carbonmaterial, it is possible to reduce the thickness of a coating. Thismakes it possible to easily adjust the height of the protrusions 43 a,43 b, 53 a and 53 b to a desired value.

The upper fixed plate 2 a is made of an insulating material, e.g.,glass, and is provided at the side of the movable electrodes 4 a and 5a, namely above the sensor chip 1 in the illustrated example. On thelower surface of the upper fixed plate 2 a, first and second fixedelectrodes 20 a and 20 b are arranged side by side in the x-direction insuch positions as to face the weight portion 4 (the movable electrode 4a) of the sensor chip 1 along the up-down direction. First and secondfixed electrodes 21 a and 21 b are arranged side by side in thex-direction in such positions as to face the weight portion 5 (themovable electrode 5 a) of the sensor chip 1 along the up-down direction.At one x-direction end side of the upper fixed plate 2 a, fivethrough-holes 22 a through 22 e are arranged in the y direction topenetrate through the upper fixed plate 2 a. On the lower surface of theupper fixed plate 2 a, there is formed a plurality of conductivepatterns (not shown) electrically connected to the respective fixedelectrodes 20 a, 20 b, 21 a and 21 b.

On the other hand, four electrode portions 8 a, 8 b, 9 a and 9 b spacedapart from the frame portion 3 are arranged side by side at onex-direction end side of the sensor chip 1. Detection electrodes 80 a, 80b, 90 a and 90 b made of metal films are formed substantially at thecenters of the upper surfaces of the four electrode portions 8 a, 8 b, 9a and 9 b. Pressure contact electrodes 81 a, 81 b, 91 a and 91 b (onlythe pressure contact electrode 91 a is shown in the drawings) made ofmetal films are formed on the upper surfaces of the end sections of thefour electrode portions 8 a, 8 b, 9 a and 9 b facing the rims 3 a and 3b. The detection electrode 80 a (80 b) and the pressure contactelectrode 81 a (81 b) are connected to each other. An earth electrode 10is formed on the upper surface of the frame portion 3 between theelectrode portions 8 b and 9 a. The earth electrode 10 is electricallyconnected to the movable electrode 4 a through the beam portions 6 a and6 b and to the movable electrode 5 a through the beam portions 7 a and 7b. If the upper fixed plate 2 a is bonded to the upper surface of thesensor chip 1, the conductive patterns formed on the lower surface ofthe upper fixed plate 2 a are connected, by pressure contact, to thepressure contact electrodes 81 a, 81 b, 91 a and 91 b. Thus therespective detection electrodes 80 a, 80 b, 90 a and 90 b areelectrically connected to the fixed electrodes 20 a, 20 b, 21 a and 21 band are exposed to the outside through the through-holes 22 a through 22d of the upper fixed plate 2 a. The earth electrode 10 is also exposedto the outside through the through-hole 22 e.

In the present embodiment, as shown in FIG. 2, gaps are provided betweenthe electrode portions 8 a and 8 b, between the electrode portions 9 aand 9 b, between the electrode portions 8 a, 8 b, 9 a and 9 b and theframe portion 3 and between the electrode portions 8 a, 8 b, 9 a and 9 band the weight portions 4 and 5. With this configuration, the respectivedetection electrodes 80 a, 80 b, 90 a and 90 b are electricallyinsulated from one another. It is therefore possible to reduce theparasitic capacitance of the detection electrodes 80 a, 80 b, 90 a and90 b and the crosstalk between the electrodes, which makes it possibleto perform accurate detection of capacitance.

Just like the upper fixed plate 2 a, the lower fixed plate 2 b is madeof an insulating material such as glass or the like. The lower fixedplate 2 b is provided at the opposite side of the sensor chip 1 from theupper fixed plate 2 a, namely below the sensor chip 1.Adherence-preventing films 23 a and 23 b are formed on the upper surfaceof the lower fixed plate 2 b in such positions as to face the weightportions 4 and 5 of the sensor chip 1 along the up-down direction. Theadherence-preventing films 23 a and 23 b are made of the same materialas the fixed electrodes 20 a, 20 b, 21 a and 21 b, e.g., aluminum-basedalloy. The adherence-preventing films 23 a and 23 b serve to prevent thelower surfaces of the rotated weight portions 4 and 5 from adhering tothe lower fixed plate 2 b. If the adherence-preventing films 23 a and 23b are made of the same material as the fixed electrodes 20 a, 20 b, 21 aand 21 b in this manner, it becomes possible to easily form theadherence-preventing films 23 a and 23 b. At this time, if theadherence-preventing films 23 a and 23 b and the fixed electrodes 20 a,20 b, 21 a and 21 b are formed simultaneously, it is possible toaccurately set the distance between the weight portions 4 and 5 and thefixed electrodes 20 a, 20 b, 21 a and 21 b and the distance between theweight portions 4 and 5 and the lower fixed plate 2 b.

If the adherence-preventing films 23 a and 23 b are formed through asemiconductor manufacturing process, fine irregularities are left on thesurfaces of the adherence-preventing films 23 a and 23 b. This makes itpossible to more reliably prevent the weight portions 4 and 5 fromadhering to the lower fixed plate 2 b. In this regard, if theadherence-preventing films 23 a and 23 b are made of aluminum-basedalloy, it becomes easy to perform etching. Short-circuit between theadherence-preventing films 23 a and 23 b and the weight portions 4 and 5may be prevented by forming an organic thin film, e.g., a polyimide thinfilm, which is highly compatible with a semiconductor manufacturingprocess and easy to process, on the surfaces of the adherence-preventingfilms 23 a and 23 b.

In the present embodiment, the rim 3 a, the weight portion 4, the beamportions 6 a and 6 b, the movable electrode 4 a, the first and secondfixed electrodes 20 a and 20 b and the detection electrodes 80 a and 80b make up one sensor unit. The rim 3 b, the weight portion 5, the beamportions 7 a and 7 b, the movable electrode 5 a, the first and secondfixed electrodes 21 a and 21 b and the detection electrodes 90 a and 90b make up another sensor unit. Two sensor units are one-piece formedwith each other in a state that the orientations of the weight portions4 and 5 (the arrangements of the solid sections 40 and 50 and the recesssections 41 and 51) are 180 degrees inverted on the same plane.

Description will now be made on the detection operation of the presentembodiment. First, it is assumed that acceleration is applied to theweight portion 4 in the x-direction. If acceleration is applied in thex-direction, the weight portion 4 rotates about the rotation axisthereof, thereby changing the distances between the movable electrode 4a and the first and second fixed electrodes 20 a and 20 b. As a result,the capacitances C1 and C2 between the movable electrode 4 a and therespective fixed electrodes 20 a and 20 b are also changed. In theregard, the capacitances C1 and C2 at the time of application ofacceleration in the x-direction can be represented by equations:

C1=C0−ΔC  (1); and

C2=C0+ΔC  (2),

where C0 denotes the capacitance between the movable electrode 4 a andthe respective fixed electrodes 20 a and 20 b when acceleration is notapplied in the x-direction and ΔC denotes the capacitance changegenerated by the application of acceleration.

Similarly, the capacitances C3 and C4 between the movable electrode 5 aand the respective fixed electrodes 21 a and 21 b at the time ofapplication of acceleration to the weight portion 5 in the x-directioncan be represented by equations:

C3=C0−ΔC  (3); and

C4=C0+ΔC  (4).

In this connection, the values of the capacitances C1 through C4 can bedetected by arithmetically processing the voltage signals extracted fromthe detection electrodes 80 a, 80 b, 90 a and 90 b. Then, the sum (±4ΔC)of a differential value CA (=C1-C2) between the capacitances C1 and C2acquired from one of the sensor units and a differential value CB(=C3-C4) between the capacitances C3 and C4 acquired from the othersensor unit is calculated. Based on the sum of the differential valuesCA and CB, it is possible to calculate the direction and magnitude ofthe acceleration applied in the x-direction.

Next, it is assumed that acceleration is applied to the weight portion 4in the z-direction. If acceleration is applied in the z-direction, theweight portion 4 rotates about the rotation axis thereof, therebychanging the distances between the movable electrode 4 a and the firstand second fixed electrodes 20 a and 20 b. As a result, the capacitancesC1 and C2 between the movable electrode 4 a and the respective fixedelectrodes 20 a and 20 b are also changed. In the regard, thecapacitances C1′ and C2′ at the time of application of acceleration inthe z-direction can be represented by equations:

C1′=C0′−ΔC′  (5); and

C2′=C0′+ΔC′  (6),

where C0′ denotes the capacitance between the movable electrode 4 a andthe respective fixed electrodes 20 a and 20 b when acceleration is notapplied in the z-direction and ΔC′ denotes the capacitance changegenerated by the application of acceleration.

Similarly, the capacitances C3′ and C4′ between the movable electrode 5a and the respective fixed electrodes 21 a and 21 b at the time ofapplication of acceleration to the weight portion 5 in the z-directioncan be represented by equations:

C3′=C0′−ΔC′  (7); and

C4′=C0′+ΔC′  (8).

Then, the difference (±4ΔC′) of a differential value CA′ (=C1′−C2′)between the capacitances C1′ and C2′ acquired from one of the sensorunits and a differential value CB′ (=C3′−C4′) between the capacitancesC3′ and C4′ acquired from the other sensor unit is calculated. Based onthe difference of the differential values CA′ and CB′, it is possible tocalculate the direction and magnitude of the acceleration applied in thez-direction. The arithmetic processing for finding the direction andmagnitude of the acceleration applied in the x-direction and thez-direction using the sum of the differential values CA and CB and thedifference of the differential values CA′ and CB′ is well-known in theart and, therefore, will not described in detail herein.

In the event that the area of the movable electrode 4 a or 5 a isincreased with a view to enhance the detection sensitivity of theacceleration sensor as set forth above, it may be possible to adopt amethod in which the thickness of the weight portion 4 or 5 is increasedso that the angle between the perpendicular line extending from thegravity center position of the weight portion 4 or 5 to the rotationaxis and the surface of the movable electrode 4 a or 5 a can becomesubstantially equal to 45 degrees. It may also be possible to adopt amethod in which the section of the weight portion 4 or 5 existing justbelow the beam portions 6 a, 6 b, 7 a and 7 b is cut away to reduce theweight of the weight portion 4 or 5. However, these methods are notdesirable. In the present embodiment, as shown in FIG. 1, the beamportions 6 a, 6 b, 7 a and 7 b (only the beam portion 6 b is shown inFIG. 1) are shifted from the generally longitudinal center of the weightportion 4 or 5 toward the recess section 41 or 51 (toward the rightside) so that the angle θ between the perpendicular line extending fromthe gravity center position of the weight portion 4 or 5 to the rotationaxis and the surface of the movable electrode 4 a or 5 a can becomesubstantially equal to 45 degrees. Since the angle θ can be kept atabout 45 degrees by merely shifting the beam portions 6 a, 6 b, 7 a and7 b, it is possible to enhance the detection sensitivity without havingto increase the thickness of the weight portion 4 or 5 or to reduce theweight of the weight portion 4 or 5.

In the present embodiment, the operation of the acceleration sensor canbe confirmed in the below-mentioned order. More specifically, the weightportions 4 and 5 are rotated by generating an attraction force betweenthe first fixed electrode 20 a or the second fixed electrode 20 b andthe movable electrode 4 a or between the first fixed electrode 21 a orthe second fixed electrode 21 b and the movable electrode 5 a. Thenormal operation of the acceleration sensor can be confirmed bydetecting the change in capacitance between the fixed electrodes 20 a,20 b, 21 a and 21 b and the weight portions 4 and 5 caused by therotation of the weight portions 4 and 5. Alternatively, the operation ofthe acceleration sensor may be confirmed by generating an attractionforce between the adherence-preventing films 23 a and 23 b and themovable electrodes 4 a and 5 a.

In the present embodiment, the acceleration sensor for detectingacceleration in two directions, i.e., in the x-direction and thez-direction, has been described by way of example. Alternatively, if asensor unit including a weight portion 4 in which the recess section 41is not formed is rotated 90 degrees within the x-y plane and arranged ina symmetrical relationship with respect to the other sensor unit, it ispossible to realize an acceleration sensor capable of detectingacceleration in three directions including the y-direction.

Second Embodiment

Hereinafter, an acceleration sensor according to a second embodiment ofthe present embodiment will now be described with reference to thedrawings. The basic configuration of the present embodiment is common tothe first embodiment. Common components will be designated by likereference symbols with no description made thereon. The accelerationsensor of the present embodiment remains substantially the same as theconfiguration shown in FIG. 2 except that each of the weight portions isconfigured as shown in FIG. 6.

As shown in FIG. 6, each of the weight portions 4 and 5 includes a firstrecess section 41 or 51 opened in one surface (the lower surface)thereof and a first solid section 40 or 50 one-piece formed with therecess section 41 or 51. The first recess section 41 or 51 is formed tohave a rectangular shape when seen in a plan view in the directionnormal to the open surface (in the up-down direction). A reinforcingwall 42 or 52 for dividing the inside of the recess section 41 or 51into two spaces is one-piece formed with each of the weight portions 4and 5.

In the present embodiment, as shown in FIG. 6, a second recess section44 or 54 (only the second recess section 44 is shown in FIG. 6) openedin one surface (the lower surface) thereof is formed in the solidsection 40 or 50. An auxiliary weight portion 45 or 55 (only theauxiliary weight portion 45 is shown in FIG. 6) made of a metallicmaterial higher in specific gravity than the material of the weightportion 4 or 5 is embedded in the second recess section 44 or 54. If theweight portion 4 or 5 is made of silicon having a specific gravity of2.33 g/cm³, it is preferred that the constituent material of theauxiliary weight portion 45 or 55 be nickel (having a specific gravityof 8.90 g/cm³), tungsten (having a specific gravity of 19.3 g/cm³),chromium (having a specific gravity of 7.87 g/cm³), palladium (having aspecific gravity of 12.02 g/cm³), platinum (having a specific gravity of21.45 g/cm³) or manganese (having a specific gravity of 7.43 g/cm³). Itis preferred that the weight of the auxiliary weight portion 45 or 55 besubstantially equal to the weight of a structural body making up theouter wall of the first recess section 41 or 51.

In the event that, as in the conventional acceleration sensor, the areaof the movable electrode 4 a or 5 a is increased with a view to enhancethe detection sensitivity of the acceleration sensor, it may be possibleto adopt a method in which the thickness of the weight portion 4 or 5 isincreased so that the angle between the perpendicular line extendingfrom the gravity center position of the weight portion 4 or 5 to therotation axis and the surface of the movable electrode 4 a or 5 a canbecome substantially equal to 45 degrees. It may also be possible toadopt a method in which the section of the weight portion 4 or 5existing just below the beam portions 6 a, 6 b, 7 a and 7 b is cut awayto reduce the weight of the weight portion 4 or 5. However, thesemethods are not desirable. In the present embodiment, as shown in FIG.6, the beam portions 6 a, 6 b, 7 a and 7 b (only the beam portion 6 b isshown in FIG. 6) are shifted from the substantially longitudinal centerof the weight portion 4 or 5 toward the recess section 41 or 51 (towardthe right side) so that the angle θ between the perpendicular lineextending from the gravity center position of the weight portion 4 or 5to the rotation axis and the surface of the movable electrode 4 a or 5 acan become substantially equal to 45 degrees. Since the angle θ can bekept at about 45 degrees by merely shifting the beam portions 6 a, 6 b,7 a and 7 b, it is possible to enhance the detection sensitivity withouthaving to increase the thickness of the weight portion 4 or 5 or toreduce the weight of the weight portion 4 or 5.

In the present embodiment, the second recess section 44 or 54 is formedin the solid section 40 or 50 of the weight portion 4 or 5. Theauxiliary weight portion 45 or 55 is embedded in the second recesssection 44 or 54. It is therefore possible to reduce the size of theweight portion 4 or 5 while maintaining the weight balance of the weightportion 4 or 5. Consequently, it is possible to reduce the overall sizeof the acceleration sensor.

In the present embodiment, the operation of the acceleration sensor canbe confirmed in the below-mentioned order. More specifically, the weightportions 4 and 5 are rotated by generating an attraction force betweenthe first fixed electrode 20 a or the second fixed electrode 20 b andthe movable electrode 4 a or between the first fixed electrode 21 a orthe second fixed electrode 21 b and the movable electrode 5 a. Thenormal operation of the acceleration sensor can be confirmed bydetecting the change in capacitance between the fixed electrodes 20 a,20 b, 21 a and 21 b and the weight portions 4 and 5 caused by therotation of the weight portions 4 and 5. Alternatively, the operation ofthe acceleration sensor may be confirmed by generating an attractionforce between the adherence-preventing films 23 a and 23 b and themovable electrodes 4 a and 5 a.

In the present embodiment, just like the first embodiment, theacceleration sensor for detecting acceleration in two directions, i.e.,in the x-direction and the z-direction, has been described by way ofexample. Alternatively, if a sensor unit including a weight portion 4 inwhich the recess section 41 is not formed is rotated 90 degrees withinthe x-y plane and arranged in a symmetrical relationship with respect tothe other sensor unit, it is possible to realize an acceleration sensorcapable of detecting acceleration in three directions including they-direction.

Next, acceleration sensors according to third and fourth embodiments ofthe present embodiment will be described with reference to the drawings.The basic configurations of the third and fourth embodiments are commonto the first embodiment. Common components will be designated by likereference symbols with no description made thereon. In the followingdescription, the vertical direction and the horizontal direction in FIG.7A will be defined as an up-down direction and a left-right direction.In the third and fourth embodiments, the upper fixed plate 2 acorresponds to a “first fixed plate” and the lower fixed plate 2 bcorresponds to a “second fixed plate”.

Third Embodiment

The present embodiment is characterized in that, as shown in FIG. 7A,engraving sections 20 c, 20 d, 21 c and 21 d are formed in the areas ofthe fixed electrodes 20 a, 20 b, 21 a and 21 b facing the protrusions 43a and 53 a by digging out one surface (the lower surface) of the upperfixed plate 2 a. Therefore, even if an impact great enough to bring theprotrusions 43 a and 53 a into contact with the fixed electrodes 20 a,20 b, 21 a and 21 b is applied to the acceleration sensor, theprotrusions 43 a and 53 a come into contact with the upper fixed plate 2a through the engraving sections 20 c, 20 d, 21 c and 21 d. Thus theprotrusions 43 a and 53 a do not make direct contact with the fixedelectrodes 20 a, 20 b, 21 a and 21 b. It is therefore possible toprevent the protrusions 43 a and 53 a from adhering to the fixedelectrodes 20 a, 20 b, 21 a and 21 b.

The respective fixed plates 2 a and 2 b are made of a glass material andthe protrusions 43 a and 53 a are formed of a silicon film or a siliconoxide film. Therefore, it is quite unlikely that the respective fixedplates 2 a and 2 b and the protrusions 43 a and 53 a adhere to eachother even when they collide with each other. However, it cannot bedefinitely said that the respective fixed plates 2 a and 2 b and theprotrusions 43 a and 53 a never adhere to each other. In light of this,it is preferred that, as shown in FIG. 7B, the areas of one surface ofthe respective fixed plates 2 a and 2 b corresponding to the engravingsections 20 c, 20 d, 21 c and 21 d be roughened to have fine surfaceirregularities. This makes it possible to prevent the protrusions 43 aand 53 a from adhering to the respective fixed plates 2 a and 2 b.Examples of the method of roughening one surface of the respective fixedplates 2 a and 2 b include sand blasting, wet etching using such aliquid as an aqueous solution of hydrofluoric acid and dry etching usingsuch a gas as carbon tetra fluoride.

Fourth Embodiment

The present embodiment is characterized in that, as shown in FIG. 8A,thin films A made of a material higher in hardness than the constituentmaterial of the protrusions 43 a and 53 a are provided on the surfacesof the protrusions 43 a and 53 a of the third embodiment. The thin filmsA are made of, e.g., a silicon nitride film, which is higher in hardnessthan a silicon film or a silicon oxide film. While the silicon nitridefilm is higher in hardness than the silicon oxide film, cracks aregenerated in the silicon nitride film due to the internal stressesthereof if the silicon nitride film is formed thick (to have a thicknessof 0.2 μm or more). In the present embodiment, the protrusions 43 a and53 a are formed into a thickness of 1 to 2 μm using a silicon film or asilicon oxide film as a base material. The thin films A having athickness of 0.2 μm or less are formed on the surfaces of theprotrusions 43 a and 53 a.

With the configuration described above, it is possible to reliablyprevent the protrusions 43 a and 53 a from adhering to the respectivefixed plates 2 a and 2 b. Since the mechanical strength of theprotrusions 43 a and 53 a grows higher, it is possible to prevent theprotrusions 43 a and 53 a from being damaged by the collision with therespective fixed plates 2 a and 2 b. The constituent material of thethin films A is not limited to the silicon nitride film but may be,e.g., a carbon material. If carbon nano tubes are used as the carbonmaterial, it becomes possible to reduce the thickness of the thin filmsA and to easily adjust the thickness of the protrusions 43 a and 53 a toa desired value.

In the present embodiment, just like the third embodiment, it ispreferred that the areas of one surface of the respective fixed plates 2a and 2 b corresponding to the engraving sections 20 c, 20 d, 21 c and21 d be roughened to have fine surface irregularities (see FIG. 85).

While the protrusions 43 a and 53 a are formed in the respective movableelectrodes 4 a and 5 a in the third and fourth embodiments, theprotrusions 43 a and 53 a may be formed in the fixed electrodes 20 a, 20b, 21 a and 21 b as shown in FIG. 9. In this case, if an impact isapplied to the acceleration sensor, the protrusions 43 a and 53 a comeinto contact with the movable electrodes 4 a and 5 a. Therefore, thereis no possibility that the fixed electrodes 20 a, 20 b, 21 a and 21 band the movable electrodes 4 a and 5 a make direct contact with eachother. Accordingly, it is possible to prevent the movable electrodes 4 aand 5 a from adhering to the fixed electrodes 20 a, 20 b, 21 a and 21 b.

In the third and fourth embodiments, gaps are provided between theelectrode portions 8 a, 8 b, 9 a, 9 b and 10 a adjoining to each other,between the electrode portions 8 a, 8 b, 9 a, 9 b and 10 a and the frameportion 3 and between the electrode portions 8 a, 8 b, 9 a, 9 b and 10 aand the weight portions 4 and 5. With this configuration, the respectivedetection electrodes 80 a, 80 b, 90 a and 90 b are electricallyinsulated from one another. It is therefore possible to reduce theparasitic capacitance of the detection electrodes 80 a, 80 b, 90 a and90 b and the crosstalk between the electrodes, which makes it possibleto perform accurate detection of capacitance.

If the adherence-preventing films 23 a and 23 b are made ofaluminum-based metal as in the prior art and are formed through asemiconductor manufacturing process, fine irregularities are left on thesurfaces of the adherence-preventing films 23 a and 23 b. This makes itpossible to reliably prevent the weight portions 4 and 5 and theprotrusions 43 b and 53 b from adhering to the lower fixed plate 2 b.However, aluminum is one of relatively soft metals. Therefore, ifcollision occurs repeatedly, the surfaces of the adherence-preventingfilms 23 a and 23 b become flat and the contact area grows larger. Thisposes a problem in that the weight portions 4 and 5 and the protrusions43 b and 53 b become easy to adhere to the lower fixed plate 2 b. In thethird and fourth embodiments, the adherence-preventing films 23 a and 23b are made of a material having substantially the same hardness as theweight portions 4 and 5 and the protrusions 43 b and 53 b, therebypreventing one of the protrusions 43 b and 53 b and the lower fixedplate 2 b from being deformed by collision. Consequently, it is possibleto appropriately prevent the weight portions 4 and 5 and the protrusions43 b and 53 b from adhering to the lower fixed plate 2 b.

In the present embodiment, just like the first embodiment, the operationof the acceleration sensor can be confirmed in the below-mentionedorder. More specifically, the weight portions 4 and 5 are rotated bygenerating an attraction force between the first fixed electrode 20 a orthe second fixed electrode 20 b and the movable electrode 4 a or betweenthe first fixed electrode 21 a or the second fixed electrode 21 b andthe movable electrode 5 a. The normal operation of the accelerationsensor can be confirmed by detecting the change in capacitance betweenthe fixed electrodes 20 a, 20 b, 21 a and 21 b and the weight portions 4and 5 caused by the rotation of the weight portions 4 and 5.Alternatively, the operation of the acceleration sensor may be confirmedby generating an attraction force between the adherence-preventing films23 a and 23 b and the movable electrodes 4 a and 5 a.

Fifth Embodiment

An acceleration sensor according to a fifth embodiment of the presentembodiment will now be described with reference to the drawings. Thebasic configuration of the present embodiment is common to the firstembodiment. Common components will be designated by like referencesymbols with no description made thereon. In the following description,the vertical direction in FIG. 10 will be defined as an up-downdirection, the direction parallel to the longitudinal direction of asensor chip 1 as an x-direction, the direction parallel to thetransverse direction of the sensor chip 1 as a y-direction and thedirection orthogonal to the x-direction and the y-direction as az-direction.

In the present embodiment, as shown in FIG. 10, the respective electrodeportions 8 a, 8 b, 9 a, 9 b and 10 a are linearly arranged along thex-direction substantially at the transverse (y-direction) center of thesensor chip 1. In other words, the respective electrode portions 8 a, 8b, 9 a, 9 b and 10 a are arranged to divide the sensor chip 1 into twohalves. An earth electrode 10 is provided on the upper surface of theelectrode portion 10 a. The weight portions 4 and 5 are arranged inpoint symmetry with respect to the center of the array of the electrodeportions 8 a, 8 b, 9 a, 9 b and 10 a. The respective beam portions 6 a,6 b, 7 a and 7 b are arranged so that the straight line interconnectingthe beam portions 6 a, 6 b, 7 a and 7 b can extend in the direction(y-direction) orthogonal to the arranging direction of the electrodeportions 8 a, 8 b, 9 a, and 9 b. The electrode portion 8 a and the firstfixed electrode 20 a are electrically connected to each other throughthe conductive pattern. The electrode portion 8 b and the second fixedelectrode 20 b are electrically connected to each other through theconductive pattern. The electrode portion 9 a and the first fixedelectrode 21 a are electrically connected to each other through theconductive pattern. The electrode portion 9 b and the second fixedelectrode 21 b are electrically connected to each other through theconductive pattern. In the present embodiment, as shown in FIG. 11, tworeinforcing walls 42 and 52 are one-piece formed with each of the weightportions 4 and 5 so as to divide the recess section 41 or 51 of each ofthe weight portions 4 and 5 into three spaces.

With the configuration described above, the sensor chip 1 of the presentembodiment is in point symmetry with respect to the earth electrode 10.This enhances the symmetry of the acceleration sensor as a whole. Evenwhen the acceleration sensor is distorted by thermal expansion or othercauses, the distortion is uniformly generated in the entirety of theacceleration sensor. Thus the overall balance is not impaired.Accordingly, a difference is hardly generated between the stressesconcentrating on the solid section 40 or 50 and the recess section 41 or51 of each of the weight portions 4 and 5 and the beam portions 6 a, 6b, 7 a and 7 b. It is therefore possible to increase the accuracy of theoutput temperature characteristics.

In the present embodiment, the distances between the respectiveelectrode portions 8 a, 8 b, 9 a, 9 b and 10 a and the respective weightportions 4 and 5 become equal to each other and, therefore, thedistances between the respective electrode portions 8 a, 8 b, 9 a, 9 band 10 a and the fixed electrodes 20 a, 20 b, 21 a and 21 b getequalized. This makes it possible to equalize the wiring lengths of therespective conductive patterns interconnecting the electrodes.Accordingly, it is possible to reduce the difference in parasiticcapacitance of the respective conductive patterns and to reduce thedifference in capacitance between the movable electrodes 4 a and 5 a andthe fixed electrodes 20 a, 20 b, 21 a and 21 b.

In the present embodiment, it is possible to increase the distance fromthe beam portions 6 a, 6 b, 7 a and 7 b to the longitudinal ends of theweight portions 4 and 5, namely the rotation radii of the weightportions 4 and 5. This makes it possible to reduce the rotationaldisplacement required for the sensor chip 1 to obtain the same detectionsensitivity as provided by the conventional acceleration sensor of thesame size. Accordingly, it is possible to increase the bending strengthof the beam portions 6 a, 6 b, 7 a and 7 b. Even if the movableelectrodes 4 a and 5 a adhere to the fixed electrodes 20 a, 20 b, 21 aand 21 b, it is possible to detach the movable electrodes 4 a and 5 afrom the fixed electrodes 20 a, 20 b, 21 a and 21 b using therestoration force of the beam portions 6 a, 6 b, 7 a and 7 b.

In the present embodiment, just like the first embodiment, it becomeseasy to form the protrusions 43 a, 43 b, 53 a and 53 b if theprotrusions 43 a, 43 b, 53 a and 53 b are formed of the main material ofthe sensor chip 1 such as a silicon film or a silicon oxide film asshown in FIG. 12. The surface layers of the protrusions 43 a, 43 b, 53 aand 53 b may be coated with a carbon material. In this case, it ispossible to increase the mechanical strength of the protrusions 43 a, 43b, 53 a and 53 b and to prevent the protrusions 43 a, 43 b, 53 a and 53b from being damaged by the collision with the upper fixed plate 2 a andthe lower fixed plate 2 b. If carbon nano tubes are used as the carbonmaterial, it is possible to reduce the thickness of a coating. Thismakes it possible to easily adjust the height of the protrusions 43 a,43 b, 53 a and 53 b to a desired value.

In the present embodiment, just like the first embodiment, it becomespossible to easily form the adherence-preventing films 23 a and 23 b ifthe adherence-preventing films 23 a and 23 b are made of the samematerial as the fixed electrodes 20 a, 20 b, 21 a and 21 b as shown inFIG. 10. At this time, if the adherence-preventing films 23 a and 23 band the fixed electrodes 20 a, 20 b, 21 a and 21 b are formedsimultaneously, it is possible to accurately set the distance betweenthe weight portions 4 and 5 and the fixed electrodes 20 a, 20 b, 21 aand 21 b and the distance between the weight portions 4 and 5 and thelower fixed plate 2 b.

If the adherence-preventing films 23 a and 23 b are formed through asemiconductor manufacturing process, fine irregularities are left on thesurfaces of the adherence-preventing films 23 a and 23 b. This makes itpossible to reliably prevent the weight portions 4 and 5 from adheringto the lower fixed plate 2 b. In this regard, if theadherence-preventing films 23 a and 23 b are made of aluminum-basedalloy, it becomes easy to perform etching. Short-circuit between theadherence-preventing films 23 a and 23 b and the weight portions 4 and 5may be prevented by forming an organic thin film, e.g., a polyimide thinfilm, which is highly compatible with a semiconductor manufacturingprocess and easy to process, on the surfaces of the adherence-preventingfilms 23 a and 23 b.

In the present embodiment, as shown in FIG. 10, gaps are providedbetween the electrode portions 8 a, 8 b, 9 a, 9 b and 10 a adjoining toeach other, between the electrode portions 8 a, 8 b, 9 a, 9 b and 10 aand the frame portion 3 and between the electrode portions 8 a, 8 b, 9a, 9 b and 10 a and the weight portions 4 and 5. With thisconfiguration, the respective detection electrodes 80 a, 80 b, 90 a and90 b are electrically insulated from one another. It is thereforepossible to reduce the parasitic capacitance of the detection electrodes80 a, 80 b, 90 a and 90 b and the crosstalk between the electrodes,which makes it possible to perform accurate detection of capacitance.

In the present embodiment, as shown in FIG. 12, the beam portions 6 a, 6b, 7 a and 7 b (only the beam portion 6 a is shown in FIG. 12) areshifted from the substantially longitudinal center of the weight portion4 or 5 toward the recess section 41 or 51 (toward the left side in FIG.12) so that the angle θ between the perpendicular line extending fromthe gravity center position of the weight portion 4 or to the rotationaxis and the surface of the movable electrode 4 a or 5 a can becomesubstantially equal to 45 degrees. Since the angle θ can be kept atabout 45 degrees by merely shifting the beam portions 6 a, 6 b, 7 a and7 b, it is possible to enhance the detection sensitivity without havingto increase the thickness of the weight portion 4 or 5 or to reduce theweight of the weight portion 4 or 5.

In the present embodiment, just like the first embodiment, the operationof the acceleration sensor can be confirmed in the below-mentionedorder. More specifically, the weight portions 4 and 5 are rotated bygenerating an attraction force between the first fixed electrode 20 a orthe second fixed electrode 20 b and the movable electrode 4 a or betweenthe first fixed electrode 21 a or the second fixed electrode 21 b andthe movable electrode 5 a. The normal operation of the accelerationsensor can be confirmed by detecting the change in capacitance betweenthe fixed electrodes 20 a, 20 b, 21 a and 21 b and the weight portions 4and 5 caused by the rotation of the weight portions 4 and 5.Alternatively, the operation of the acceleration sensor may be confirmedby generating an attraction force between the adherence-preventing films23 a and 23 b and the movable electrodes 4 a and 5 a.

The above-described embodiments can be appropriately combined withoutdeparting from the technical scope of the present invention.

While the invention has been shown and described with respect to theembodiments, the present invention is not limited thereto. It will beunderstood by those skilled in the art that various changes andmodifications may be made without departing from the scope of theinvention as defined in the following claims.

1. An acceleration sensor, comprising: a sensor unit including a weightportion having a recess section with one open surface and a solidsection one-piece formed with the recess section, a pair of beamportions configured to rotatably support the weight portion in such astate that the recess section and the solid section are arranged along arotation direction, a movable electrode provided on the opposite surfaceof the weight portion from the open surface of the recess section toextend over the recess section and the solid section, a first fixedelectrode arranged at the opposite side of the movable electrode fromthe recess section and a second fixed electrode arranged at the oppositeside of the movable electrode from the solid section, the accelerationsensor being configured to detect acceleration based on a change incapacitance between the movable electrode and the fixed electrodescaused by rotation of the weight portion about a rotation axis definedby a straight line interconnecting the beam portions, wherein the beamportions are shifted toward the recess section such that an anglebetween a perpendicular line extending from a gravity center position ofthe weight portion to the rotation axis and a surface of the movableelectrode becomes substantially equal to 45 degrees.
 2. The accelerationsensor of claim 1, wherein a separate recess section with one opensurface is provided in the solid section and an auxiliary weight portionmade of a metallic material is embedded in the separate recess section.3. The acceleration sensor of claim 2, wherein the auxiliary weightportion is made of a metallic material higher in specific gravity than aconstituent material of the weight portion.
 4. The acceleration sensorof claim 2, wherein the auxiliary weight portion has a weightsubstantially equal to a weight of a structural body making up an outerwall of the recess section.
 5. The acceleration sensor of claim 1,wherein acceleration applied to the weight portion in a first directionand acceleration applied to the weight portion in a second directionorthogonal to the first direction are detected.
 6. The accelerationsensor of claim 1, wherein the sensor unit includes a plurality ofsensor units formed in a single chip.
 7. The acceleration sensor ofclaim 6, wherein the sensor unit includes two sensor units formed in thesingle chip, one of the two sensor units rotated 180 degrees on a singleplane with respect to the other sensor unit.
 8. The acceleration sensorof claim 7, wherein the two sensor units are arranged adjacent to eachother.
 9. The acceleration sensor of claim 1, wherein the sensor unitincludes a frame portion surrounding the weight portion and a pair ofdetection electrodes formed in the frame portion and electricallyconnected to the fixed electrodes and wherein gaps are provided betweenthe detection electrodes, between the detection electrodes and the frameportion and between the detection electrodes and the weight portion. 10.The acceleration sensor of claim 1, wherein protrusions are formed onthe surfaces of the fixed electrodes facing the movable electrode or onthe surface of the movable electrode facing the fixed electrodes. 11.The acceleration sensor of claim 10, wherein the protrusions are formedof a silicon film or a silicon oxide film.
 12. The acceleration sensorof claim 10, wherein the protrusions have surface layers made of acarbon material.
 13. The acceleration sensor of claim 1, furthercomprising: a fixed plate arranged in a spaced-apart relationship withthe opposite surface of the weight portion from the fixed electrodes,the fixed plate including an adherence-preventing film for preventingthe weight portion from adhering to the fixed plate, theadherence-preventing film formed on the surface of the fixed platefacing the weight portion.
 14. The acceleration sensor of claim 13,wherein the adherence-preventing film is made of the same material asthe fixed electrodes.
 15. The acceleration sensor of claim 13, whereinthe change in capacitance between the first and second fixed electrodesand the movable electrode is detected by generating an attraction forcebetween the fixed electrodes and the movable electrode.
 16. Anacceleration sensor, comprising: a sensor unit including a weightportion having a recess section with one open surface and a solidsection one-piece formed with the recess section, a pair of beamportions configured to rotatably support the weight portion in such astate that the recess section and the solid section are arranged along arotation direction, a movable electrode provided on the opposite surfaceof the weight portion from the open surface of the recess section toextend over the recess section and the solid section, a first fixedelectrode arranged at the opposite side of the movable electrode fromthe recess section and a second fixed electrode arranged at the oppositeside of the movable electrode from the solid section, the accelerationsensor being configured to detect acceleration based on a change incapacitance between the movable electrode and the fixed electrodescaused by rotation of the weight portion about a rotation axis definedby a straight line interconnecting the beam portions; and a first fixedplate arranged in a spaced-apart relationship with the surface of theweight portion facing the fixed electrodes, the fixed electrodesprovided on one surface of the first fixed plate, wherein protrusionsare formed on the surface of the movable electrode facing the fixedelectrodes and wherein engraving sections are formed in the areas of thefixed electrodes facing the protrusions by digging out one surface ofthe first fixed plate.
 17. The acceleration sensor of claim 16, whereinsurface irregularities are provided in the areas of the first fixedplate corresponding to the engraving sections.
 18. The accelerationsensor of claim 16, wherein a thin film made of a material higher inhardness than a constituent material of the protrusions is provided on asurface of each of the protrusions.
 19. The acceleration sensor of claim16, further comprising: a second fixed plate arranged in a spaced-apartrelationship with the opposite surface of the weight portion from thefixed electrodes, the second fixed plate including anadherence-preventing film for preventing the weight portion fromadhering to the second fixed plate, the adherence-preventing film formedon the surface of the second fixed plate facing the weight portion. 20.The acceleration sensor of claim 16, wherein the sensor unit includes aframe portion surrounding the weight portion and a pair of detectionelectrodes formed in the frame portion and electrically connected to thefixed electrodes and wherein gaps are provided between the detectionelectrodes, between the detection electrodes and the frame portion andbetween the detection electrodes and the weight portion.
 21. Anacceleration sensor, comprising: a sensor unit including a weightportion having a recess section with one open surface and a solidsection one-piece formed with the recess section, a pair of beamportions configured to rotatably support the weight portion in such astate that the recess section and the solid section are arranged along arotation direction, a movable electrode provided on the opposite surfaceof the weight portion from the open surface of the recess section toextend over the recess section and the solid section, a first fixedelectrode arranged at the opposite side of the movable electrode fromthe first recess section and a second fixed electrode arranged at theopposite side of the movable electrode from the solid section, theacceleration sensor being configured to detect acceleration based on achange in capacitance between the movable electrode and the fixedelectrodes caused by rotation of the weight portion about a rotationaxis defined by a straight line interconnecting the beam portions,wherein protrusions are formed on the surfaces of the fixed electrodesfacing the movable electrode.
 22. An acceleration sensor, comprising: asensor unit including a weight portion having a recess section with oneopen surface and a solid section one-piece formed with the recesssection, a pair of beam portions configured to rotatably support theweight portion in such a state that the recess section and the solidsection are arranged along a rotation direction, a movable electrodeprovided on the opposite surface of the weight portion from the opensurface of the recess section to extend over the recess section and thesolid section, a first fixed electrode arranged at the opposite side ofthe movable electrode from the recess section, a second fixed electrodearranged at the opposite side of the movable electrode from the solidsection and a pair of electrode portions having detection electrodeselectrically connected to the fixed electrodes, the acceleration sensorbeing configured to detect acceleration based on a change in capacitancebetween the movable electrode and the fixed electrodes caused byrotation of the weight portion about a rotation axis defined by astraight line interconnecting the beam portions, wherein the sensor unitincludes two sensor units formed in a single chip, the electrodeportions are arranged along one direction to divide the chip into twohalves, the weight portions of the two sensor units are arranged inpoint symmetry with respect to a center of an array of the electrodeportions, and the beam portions are arranged such that a straight lineinterconnecting the beam portions extends in a direction orthogonal tothe arranging direction of the electrode portions.
 23. The accelerationsensor of claim 22, further comprising: a fixed plate arranged in aspaced-apart relationship with the opposite surface of the weightportion from the fixed electrodes, the fixed plate including anadherence-preventing film for preventing the weight portion fromadhering to the fixed plate, the adherence-preventing film formed on thesurface of the fixed plate facing the weight portion.
 24. Theacceleration sensor of claim 23, wherein the adherence-preventing filmis made of the same material as the fixed electrodes.